1. Field of the Invention
The present invention relates to transducer devices and more particularly to a broadband, lightweight Rayleigh wave transducer suitable for structural calibration, locating structural emission sources and detecting near-surface flaws using pulse-echo techniques.
2. Description of the Prior Art
Acoustic emission structural calibration is commonly performed either with a narrowband piezoelectric source or with a broadband mechanical source, such as the breaking of a glass capillary tube or a pencil lead. Narrowband sources however do not permit good pulse arrival time resolution. Broadband mechanical sources are very inconvenient to use because triggering is difficult to set up, the pulse shape and amplitude are not repeatable, the pulse repetition rate is very low and a transient-capture device is required to record the waveform. A piezoelectric thickness vibrator can be made broadband if an appropriate damping material is applied to one of the vibrator faces. However the search for an effective damping material is usually a time-consuming, cut-and-try process.
Acoustic emission technology usually involves the generation and reception of Rayleigh wave pulses on a solid surface. Rayleigh waves can be produced and detected by transducers whose motion is normal to the surface, but this method requires that the lateral dimensions be smaller than the Rayleigh wavelength. This is a severe restriction at high frequencies, i.e., &gt;1 MHz, resulting in low capacitance and sensitivity. Rayleigh waves can be effectively generated or received over larger areas by interdigital transducers and by angle-beam transducers. Interdigital transducers are extensively used in surface-acoustic-wave devices but are inherently narrowband, because the interdigital spacing is fixed at one-half the wavelength at the frequency of interest. Present angle-beam transducers utilize a thickness-mode transducer element coupled to a plastic wedge inclined at the critical angle for Rayleigh waves in the material to be examined. Such angle-beam transducers however are normally narrowband devices due to impedance mismatches between the element and the plastic wedge.